The present disclosure relates to vehicle exhaust systems for treating exhaust gas and more particularly, to systems and methods for improving the NOx reduction of exhaust gas.
In vehicles such as trucks, exhaust gas (which is a combination of gas and particulate matter) is processed in multiple stages prior to being released to the atmosphere as illustrated in FIG. 1. Exhaust from an engine 110 may be processed by a diesel oxidation catalyst (DOC) 120 to remove hydrocarbons, by a diesel particulate filter (DPF) 130 to remove particulate matter, and by a selective catalytic reduction device (SCR) 140 to reduce NOx to Nitrogen gas and water vapor. For the SCR 140 stage, diesel exhaust fluid (DEF) is injected into and mixed with the exhaust upstream of the SCR device 140.
Currently, trucks have an onboard tank that contains DEF. DEF is composed of approximately 32.5% Urea and 67.5% demineralized water. In most DEF injection systems, DEF is delivered through an injector. DEF delivering injectors can be of a plurality of designs which rely on a pressure gradient across an orifice (high pressure in the injector, low pressure on the outside) to atomize the fluid. DEF is injected by applying approximately nine bar pressure to the fluid. The pressure forces the fluid through the orifices into the exhaust where it is then atomized. In other systems, DEF is mixed with compressed air before it enters the injection nozzle to improve atomization.
It is desirable to have smaller droplet sizes (of the DEF) during atomization to increase the efficiency and effectiveness of the after treatment process of the exhaust gas or fluid.